1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a zoom lens system preferable for a projection optical system of a mobile liquid crystal projector.
2. Related Background Art
Up to now, various liquid crystal projectors (image projection apparatuses) have been proposed which use display elements, such as liquid crystal display elements, and project images based on the display elements onto screen surfaces.
In particular, such liquid crystal projectors have been widely used as apparatuses for projecting images formed by personal computers and the like onto large screens for conferences, presentations, and the like.
The following have recently been required for the liquid crystal projectors.
(1) In a three-panel projector using three liquid crystal display elements for three colors that are red (R), green (G), and blue (B), a mount space for optical elements, such as a polarizing plate and a dichroic prism serving as a color combining means for combining color light beams having passed through the liquid crystal display elements, must be provided between the liquid crystal display elements and a projection lens, and hence a predetermined back focal length is required.
(2) In order to minimize the influence of angle dependence of a color combining film used for the color combining means and in order to ensure a favorable pupil matching property with respect to an illumination system, a so-called telecentric optical system must be accomplished such that the pupil on the liquid crystal display element (reduction) side is located at infinity.
(3) When images (pictures) based on the liquid crystal display elements for the three colors are combined and projected on the screen, if characters and the like look doubled, a sense of resolution and quality are deteriorated. In order to prevent this, pixels for respective colors must be properly overlaid on each other throughout the entire screen. For that purpose, color misregistration (chromatic aberration of magnification) caused by the projection lens must be properly corrected throughout the entire visible region (the lens system must be an apochromat system).
(4) In order to prevent a contour portion of a projected image from being distorted undesirably, distortion must be properly corrected (in particular, if an abrupt distortion change or the like remains in a peripheral portion or an intermediate portion of an image, for instance, image quality will be degraded and thus this is not preferable).
(5) In consideration of mobility of a projector equipped with a compact panel, reductions in apparatus size and weight as well as increases in luminance of a screen and resolution of images are needed.
(6) In consideration of use environment of such a small-sized image projection apparatus, projection of a large image with a shorter projection distance must be accomplished.
Meanwhile, various zoom lenses have conventionally been proposed as projection lenses for liquid crystal projectors.
Of those zoom lenses, a retrofocus type zoom lens, in which a lens unit having negative refractive power serves as a front lens unit, is widely used for projectors as a projection lens for a wide angle of view. Aside from this, the retrofocus type zoom lens is also widely used for optical devices, such as video cameras, film cameras, and digital cameras, as a photographing lens.
For instance, Japanese Patent Application Laid-Open No. 2001-091829 proposes a projection lens for a liquid crystal projector, which is a five-unit zoom lens in which five lens units respectively having negative refractive power, positive refractive power, positive refractive power, positive refractive power, and positive refractive power are arranged in order from the enlargement side. When magnification of the zoom lens is to be changed, predetermined lens units out of the five lens units are moved as appropriate.
In this five-unit zoom lens, the first lens unit and the fifth lens unit are fixed, and the magnification change from the wide-angle end to the telephoto end is performed by moving each of the second lens unit to the fourth lens unit in the lens system to the reduction side. With this construction, a wide angle of view is realized and telecentricity is ensured while maintaining a constant entire lens length.
As for a projection lens that enlarges and projects an image displayed by a liquid crystal display element onto a screen, there is desired a lens system that achieves a large aperture and an increase in angle of view with a small size and a light weight.
When the aperture of a projection lens is increased for higher brightness and an angle of view is increased, however, aberration correction becomes difficult and the projection lens increases in size, in general. Also, when the number of lens constituting the projection lens is increased, a spectral transmission factor is lowered.
In the zoom lens proposed in Japanese Patent Application Laid-Open No. 2001-091829 described above, the F number is set at xe2x80x9c2xe2x80x9d that is large (dark), and the number of lenses constituting the entire system is around 12 that is relatively large.
Also, the material of the lens constituting the fifth lens unit added closest to the reduction conjugate side (original image side) has a small refractive index, so that aberration correction concerning off-axis luminous fluxes tends to become difficult.
The present invention provides a zoom lens system that favorably suppresses variations in various aberrations accompanying zooming and exhibits favorable optical performance throughout the entire screen while achieving reduction in size of the entire lens system.
A zoom lens system according to an aspect of the present invention includes, in order from the front (screen side in the case of a projector and object side in the case of a camera) to the rear (original image side in the case of the projector and image side in the case of the camera): a first lens unit having negative optical power; a second lens unit having positive optical power; a third lens unit having positive optical power; a fourth lens unit having positive optical power; and a fifth lens unit having positive optical power. Of these five lens units, at least three lens units move during zooming. Also, as is defined by the following conditional expression, the fifth lens unit is provided with a positive lens element having a relatively high refractive index.